You didn’t know it was there for balance? A lot of basses seem to have either 2 strap buttons or them placed lower down nowadays, Status have 2 on most of their models and Spector also place them at a lower position for balance and weight adjustment of how the actual bass sits.
It also accommodates for players who prefer their basses higher up on a strap, luthiers such as GB, assume these basses will be worn higher up because most players are using the upper register a lot. Placing the strap pin lower helps shift the basses weight creating more of a right angle or “straighter” playing position, so access to the upper register (in theory) is less strenuous, make sense?
EDIT: I also just realised this picture taken from GB Guitars Twitter page is this exact bass!